For older versions of ASP, jumping from ASP to HTML meant more strain on the server, which in turn resulted in slower forums. It doesn't affect the latest version of ASP, but it's done to increase performance for people who are stuck with older servers.
For ASP 3.0 probably none (maybe even detrimental?). For server's running ASP 2.0 (Win NT servers, basically), it will be faster than having mixed HTML / ASP files. That was the motivation behind the move.
It also keeps non-programmer webmasters away from redesigning the forums [jk]. But, half of my programming time goes to make the html code "appear" pretty-printed. No color coding help, tough especially if you have indented "if-then-else like constructs...